In a way, I could understand why Joe Musashi was not in SoR4. Whenever or not Streets or Rage and Shinobi take place on the same Earth or not, it would likely have been confusing if he cameo in the story. Maybe as a reskin or unlockable character exclusive to Arcade or Survival.
Probably. Still somehow Shinobi costume was absent from Super Monkey Ball skins where almost other franchises as popular as Shinobi got them.
There are ninja costumes but not tied to Shinobi franchise. (again, missed opportunity or may be there's a reason unknown)
Also, Roblox is... Roblox. Sonic Speed Sim has it's own isolated continuity from the main series of games so they could get away with crossovers.
And heck whenever they want human Sega characters looking like Roblox avatars or not, that's their choice.
Wish it's that simple to use another SEGA franchise to crossover with but SEGA Legal Team isn't easy to deal with.
At least that's what someone who worked in SEGA Heroes before said.
Still not as hard as Nintendo Legal Team.